Once a geosynchronous spacecraft is on-station, north-south station-keeping maneuvers consume the majority of the total propellant of the spacecraft. For geosynchronous communications spacecraft, controlling spacecraft attitude to orient communications hardware to a preselected planetary location is essential, as is periodic unloading or "desaturation" of momentum stored in the momentum wheel stabilization system of the space, craft. Therefore, increasing efficiency of north-south station-keeping, attitude correction, and momentum desaturation may significantly lengthen mission lifetime
Orbiting spacecraft may use several mechanisms for station-keeping, attitude control, and momentum desaturation The bi-propellant chemical thrusters typically used to take a spacecraft from a transfer orbit to a synchronous orbit could also be used for station-keeping, but such thrusters produce relatively powerful disturbance torques due to thruster misalignment, center of mass offsets, and plume impingements. They are, therefore, ill-suited for highly accurate attitude pointing during station-keeping, and are of almost no use in fine attitude adjustments and fine momentum desaturation. Reliability concerns also dictate that such thrusters be reserved for major spacecraft orbit and attitude error control over the lifetime of the spacecraft.
A spacecraft could use magnetic torquers for attitude control and momentum desaturation, but these devices do not assist in station-keeping since they produce torque without lateral force. Magnetic torquers change the spacecraft's attitude by using the force of an electric coil's dipole moment with respect to the earth's magnetic field. Unfortunately, magnetic torquers are heavy, provide little power, and cannot be used to control the pitch attitude of the spacecraft due to the alignment of the dipole moment section with the magnetic field of the earth.
A third possible technique is ion propulsion. In an ion propulsion thruster, an electro-magnetic field ionizes a propellant such as xenon gas, and an electrostatic field accelerates the propellant away from the thruster body. Although there is an initial weight penalty for the ion propulsion system hardware, the specific impulse of ion trusters is significantly higher than that of chemical propellants, making ion propulsion feasible for station-keeping in long missions. In addition, ion thrusters can be used to control spacecraft attitude and momentum in all three axes. For these reasons, ion thrusters are suitable and desirable for north-south station-keeping on spacecraft with long mission lifetimes.
As with chemical thrusters, ion thruster firings may produce disturbance torques on the spacecraft, resulting in undesired attitude motion. Solar pressure imparts an additional undesired disturbance on the spacecraft. Momentum wheel stabilization systems are commonly used to counteract such disturbance torques. Such systems typically include one or more momentum wheels and control loops to sense changes in the spacecraft attitude. Sensors on the spacecraft may detect yaw, pitch, and roll. The control loops determine the required speed of the wheels to absorb and off-load stored pitch and yaw momentum based on the sensed attitude. Commonly, yaw and pitch momentum are absorbed directly by the momentum wheels, while roll momentum is absorbed as a change in yaw body angle which results from the cross-coupling dynamics of momentum biased spacecraft. Momentum stored in the momentum wheels must be relieved periodically, or desaturated, to keep the momentum wheels within a finite operable speed range. Desaturation is typically accomplished by applying an external torque to the spacecraft through propulsion thrusting or magnetic torquing, to reduce the stored momentum.
Magnetic torquers cannot counteract stored momentum in one axis and are slow in the other two axes, as discussed above. Chemical thruster firings are ill-suited to maintenance of highly accurate spacecraft attitude during station-keeping because the disturbance torques they produce typically overwhelm the ability of wheel systems to maintain proper attitude. Chemical thruster firings are also typically too strong and difficult to control to use for wheel desaturation while simultaneously controlling highly accurate attitude pointing.
U.S. Pat. No. 3,937,423 to Johansen discloses a system for controlling the attitude of a vehicle along three axes with one momentum wheel of one degree of freedom, and pulsing several jets to enable correction of attitude error and dampen nutation of the vehicle while the spacecraft is on-orbit. The system of Johansen does not counteract disturbance torques or desaturate a momentum wheel system during station-keeping thrusting.
U.S. Pat. No. 4,521,855 to Lehner et al. discloses a scheme for measuring and correcting yaw error and yaw and roll momentum on a continuous on-orbit basis in an orbiting satellite, and attempts to use magnetic torquers to unload stored momentum. Lehner also references prior art helpful to the understanding of control loop and momentum distribution subsystems used in accordance with the present invention, but does not address simultaneous attitude control and momentum desaturation during north-south station-keeping maneuvers.
U.S. Pat. No. 4,767,084 to Chan et al. discloses apparatus for desaturating momentum wheels by firing thrusters from one face of the spacecraft, and then alternating firings between faces while simultaneously accomplishing automatic east-west station-keeping. Chan is helpful in referencing background art pertaining to spacecraft attitude control. Typically, east-west station-keeping requires far smaller forces than north-south station-keeping, and the disclosure of Chan et al. does not address a system for simultaneously desaturating momentum wheels while generating the relatively large lateral forces needed for north-south station-keeping. Furthermore, the disclosure of Chan et al. does not address the corresponding attitude maintenance or nutation problems.
U.S. Pat. No. 4,825,646 to Garg et al. discloses spacecraft attitude control apparatus requiring at least three pairs of thrusters. The system of Garg et al. does not relate to north-south station-keeping, nutation reduction, or momentum wheel desaturation.
U.S. Pat. No. 4,848,706 to Challoner et al. discloses an axial non-gimballed ion thruster mounted on the spun portion of a spacecraft and continuously fired for an integral number of spin periods to provide north-south velocity and attitude control. The disclosure of Challoner et al. does not address the problem of momentum wheel desaturation.
"Implementation of Electric Propulsion for North-South station keeping on the EUROSTAR Spacecraft", by T. G. Duhamel, presented at the AIAA/ASME/SAE/ASEE Twenty-fifth Joint Propulsion Conference, paper AIAA-89-2274, discloses an implementation of ion propulsion for north-south station-keeping on the EUROSTAR spacecraft. The ion thrusters are aligned through the center of mass of the spacecraft using a two-axis thruster pointing mechanism. The two-axis thrusters provide torques on both roll and yaw axes for minimizing disturbance torques caused by misalignment of thrust direction with respect to the center of mass. A pitch momentum wheel provides control around the pitch axis. The two-axis gimbals of the Duhamel disclosure do not appear to be used for three-axis attitude control, but simply to align the thrust direction with the center of mass. Duhamel does not disclose gimballing on one axis and adjusting the output of one thruster relative to another to produce torques in all three axes. Nor does the Duhamel paper disclose desaturation of the momentum wheels by producing torques. Duhamel teaches a relatively inefficient ion thruster cant of approximately forty-five degrees from the north-south direction. Furthermore, the two-axis gimbals called for by Duhamel are relatively more complex, heavy and unreliable than one-axis gimbals.
Yet another system, disclosed by Toshiba as part of an ETS-VI satellite program handout at a conference with attendance limited to employees of Toshiba, Space Systems/Loral, Inc., and Intelsat, uses four reaction wheels and chemical thruster momentum unloading for attitude stabilization during ion thrusting for east-west station-keeping. This disclosure does not teach the use of gimballed or throttled ion thrusters, nor does this disclosure teach simultaneous desaturation without chemical thrusters and attitude control during positioning maneuvers. Furthermore, the firing duration for east-west station keeping as pertains to this reference is significantly smaller than that required for north-south positioning.
Ideally, a spacecraft control system would maximize fuel efficiency during the mission lifetime without sacrificing reliability. A simple system for maintaining spacecraft attitude and desaturating momentum without independent thruster firings would realize this goal.